This can be the result of a couple different things:
1. The UFS library files might not have the correct name; please ensure the name is identical to the original file or/and ending with ".ufs".
2. The UFS library file might be corrupted; you can right-click on the file and select Verify soundbank integrity through the UVI Browser.
If you do not succeed, please download the library again from the My Products page.
3. [Mac only] The UFS file might have a permissions issue; locate the UFS file and Get Info (Cmd+I), then change privileges to Read & Write.